This invention relates to electronic ignition systems in general and more particularly to an improved circuit for obtaining spark advance in an electronic ignition system.
With the availability of better electronic components, particularly power semiconductors, electronic ignition systems replacing the old mechanical system have come into wide spread use in automobile ignition systems. Although the prior art "Kettering" system which has been used for many years works quite well, it relies on ignition points which must switch high currents. As a result, over a period of time, the points become burned. In addition, the points ride on a cam in the distributor and wear takes place changing the point gap as time goes by. As a result in order to maintain proper engine operation to maximize efficiency and minimize the emission of pollutants, periodic readjustment and replacement of the points is necessary.
Some of the earlier electronic ignition systems still used points but instead of switching the large currents necessary used the points to control semiconductor circuits which then switch the higher currents. This avoided the problem of point wear but did not overcome the problem of wear against the cam. Thus, points still needed periodic adjustment.
Present electronic ignition systems completely overcome the problems with points through the use of magnetic, photoelectric, or Hall effect pickup units in the distributor. Typical devices of this nature are disclosed for example in U.S. Pat. Nos. 3,749,974, and 3,923,030. Other systems propose utilizing crankshaft detectors. For example see U.S. Pat. Nos. 3,587,552 and 3,882,835.
There are basically two types of electronic ignition systems now is use. These are the inductive discharge system and the capacitive discharge system. The former operates much like the old "Kettering" type ignition system in that the current is switched in the primary of a coil with the secondary of the coil then providing the energy for firing the spark. The difference between the electronic ignition systems and old "Kettering" system is that rather than switching the primary through points, switching is done through semiconductors. In the capacitive discharge system the battery voltage is converted to a high voltage and this high voltage is used to charge up a capacitor which is then discharged at the firing instant.
Prior art distributors have built into them advance mechanisms which may be either or both a centrifugal and/or vacuum advance. Many electronic ignition systems utilize the prior art advance mechanism in the distributor and simply use the electronic circuits to control or supply the firing energy for the spark plugs. Although the present advance mechanisms in distributors are well developed, they are subject to wear and mechanical failure. More significantly there are limits on the types of advance curves which can be implemented and the ability to make changes in the advance curve. Thus, there have been attempts to develop electronic advanced circuits. Typical of such a circuit is that disclosed in U.S. Pat. No. 3,943,896. In the device disclosed therein, the input frequency from the distributor is converted to a voltage and that voltage used to control the timing of a monostable multivibrator, with the multivibrator then determining the firing instant. It will be recognized that such a circuit, relying as it does on an RC timing circuit in a monostable multivibrator, is difficult to control as to maximum advance. For example, the disclosed circuit relies on the points to establish maximum advance. Also, it is difficult to obtain non-linear curves and to change advance characteristics. Thus, because of these various problems many automobile manufacturers have not incorporated electronic advance into their electronic ignition systems.
Another attempt at the solution to this problem is the system disclosed in U.S. Pat. No. 3,882,835. In the device disclosed therein, outputs from a crankshaft detector feed a monostable circuit and a ramp generator. The monostable output provides inputs to a frequency to voltage generator and the ramp generator. The voltage to frequency converter generates a voltage which decreases proportionately with the frequency of the input pulse rate. The voltage to frequency converter output and ramp generator output are compared in a comparator to determine the advance firing point. In the ramp generator, another frequency to voltage converter is used to control the charge rate of the ramp generator in an attempt to achieve a ramp with nearly constant duty cycle and amplitude. The frequency to voltage converter operates by turning on an active current source for the duration of the monostable pulse.
Although in principle this system works well overcoming some of the deficiencies of the systems using variable monostable multivibrators, it too suffers from drawbacks. In the first place, it requires a relatively large number of components. More importantly, the use of a frequency to voltage converter to generate the ramp means that proper adjustment of this converter must be maintained at all times. The manner in which the frequency to voltage converter operates, relying on the monostable output to turn on an active current source can also lead to errors.
In view of these difficulties, the need for an improved advance circuit for use in electronic ignition circuits becomes evident.